Human ovarian carcinoma cells resistant to L-phenylalanine mustard have a 2-3 fold elevated content of the tripeptide glutathione as compared to drug sensitive cells. Reduction of the glutathione content by either nutritional deprivation of L-cysteine or use of DL-buthionine-S, R-sulfoximine, an ihhibitor of glutathione biosynthesis, resulted in sensitization of the resistant cell.